Various types of oscillators are commonly used to provide a reference signal for use within electronic circuits. Their piezoelectric properties allow them to be a frequency-determining element in electronic circuits. A crystal oscillator, particularly one made of quartz crystal, works by being distorted by an electric field when voltage is applied to an electrode near or on the crystal. This property is known as electrostriction or inverse piezoelectricity. When the field is removed, the quartz—which oscillates in a precise frequency—generates an electric field as it returns to its previous shape, and this can generate a voltage.
Typically, a crystal oscillation circuit includes a crystal oscillator, an inverter coupled in parallel with the crystal oscillator, and capacitors coupled to the input and output of the inverter and to ground. To conserve power, the crystal oscillation circuit includes an enable/disable mechanism. The crystal oscillator can be started by injecting energy composed of noise and/or transient power supply response. The startup time of a crystal oscillator is typically determined by the noise or transient conditions at turn-on; small-signal envelope expansion due to negative resistance; and large-signal amplitude limiting.
It is known that crystal resistance is not constant, typically being higher at start-up than when oscillating in steady state. The crystal resistance is related to the Q factor of the oscillator, which dictates the amount of power applied to the crystal to keep it oscillating at the same amplitude. As the resistance decreases, the amount of power consumed decreases. The variation in the crystal resistance causes more power to be used at start-up than is desired to achieve the best noise performance in steady state operation. However, decreasing the power such that optimal noise performance is achieved in steady state increases the amount of time for the crystal oscillator to reach steady state from start-up.
A common approach to startup the crystal oscillator is to inject high energy at the beginning, and then making the expansion even faster to reach the desired frequency. With this approach, a lot of energy is used to startup the crystal oscillator. Under this approach, the startup time for the crystal oscillator can be between 500-600 us for crystals that oscillate around 26 MHz. However, this approach does not work well in the context when power supply is limited.
Another common approach is to inject noise at the startup time into the oscillation of the crystal element. In general, certain amount of phase noise is preferred during the oscillation of the crystal element. Various techniques have been proposed to inject noises at the startup time of the crystal element.